Utility lines, such as those carrying electrical power, cable television signals or telephone signals, have traditionally been supported above ground using poles, and especially wooden poles. As used herein, the term “pole” includes various forms and definitions of elongated support members, e.g., posts and pilings, whether or not constructed of wood. Such poles must be capable of withstanding not only the columnar load applied by the weight of the objects supported thereon but also the transverse or horizontal load imposed by transverse winds or unbalanced wire tensions from angled or dead end wires that cause the upper end of the pole to deflect relative to the buried bottom end of the pole.
After some years in service, wooden utility poles tend to experience decay and rotting just below and/or slightly above ground level. While the decayed region is normally relatively small and the penetration of the decay may be limited, the pole is nonetheless structurally weakened and may not be sufficiently strong to withstand wind and other environmental factors. Under these conditions, wind forces can result in a pole breaking and toppling, sometimes without warning.
Therefore, it is necessary to periodically replace older wooden poles. The demand for replacement poles, in combination with the demand for new poles, has become increasingly difficult to meet. This demand presents environmental concerns related to deforestation and the toxic effects of preservative chemicals used to treat the poles. In addition, replacement of existing poles is expensive and may require interruption of service to users of the utility. To overcome these and other problems associated with pole replacement, various methods and apparatus for reinforcing in-service poles have been developed to extend their useful life.
One technique for reinforcing utility poles is that of coupling an elongated truss to the pole, in effect splinting or bridging across the weakened area of the pole. Such trusses are customarily adapted to extend at least partway along the pole parallel to its longitudinal axis to provide support against transverse wind forces and other loading conditions. The steel truss has been used to strengthen wooden utility poles for more than forty years.
One such pole reinforcing apparatus is the OSMOSE® Osmo-C-Truss™ system. This truss helps to restore the groundline strength of utility poles at a fraction of the cost of pole replacement. The Osmo-C-Truss™ system comprises a C-shaped galvanized steel reinforcing truss which is secured to a pole by a plurality of galvanized steel bands fastened around the perimeter of the truss/pole assembly. The Osmo-C-Truss™ system can extend the life of a pole for many years and is installed without interrupting service to utility customers.
In spite of the many advantages of the Osmo-C-Truss™ system, some performance issues are inherent in the use of a “C” or channel shaped reinforcing apparatus. One significant performance issue is related to the ability of a “C” or channel shaped design to withstand bending loads from a pole without twisting or rotating about the pole. One solution in the prior art is to increase or “beef up” the capacity of the apparatus by increasing its dimensions or the yield strength of the material of construction. However, these approaches fail to consider the underlying mechanical principles that govern the performance of such devices under load. Because the shear centers and the elastic axes of the reinforcing apparatus reside well outside the locus of the applied transverse load, there results significant torsional forces acting upon the reinforcing apparatus in addition to the expected bending forces. Specifically, “C” or channel shaped designs do not account for the relationship between the location of the shear center of the truss and the location of the transverse applied load. The further the applied load is from the shear center and elastic axis, the greater the torsional forces that act upon the truss in combination with the bending forces. Torsional forces may cause the truss to shift its position about the circumference of the pole, i.e., rotate about the pole, to a disadvantageous position wherein the truss is no longer loaded in the direction of maximum strength. Further, the reinforcing apparatus itself may twist and experience shape distortion when subjected to torsional forces, causing a reduction in performance; possibly less than the theoretical strength of the material of construction would afford.
Without a corresponding decrease in torsional rotation of the apparatus about the pole, or a reduction in the torsional forces themselves, the increased theoretical resistance to bending forces supplied by a truss having increased dimensions or higher yield material may be of little practical value. In fact, the use of higher strength materials to increase truss capacity is accompanied by a generally proportional increase in the truss rotations and deflections that occur when the truss is loaded beyond the capacity of a similarly-dimensioned truss formed of lower strength material. The reinforced truss will undergo unacceptable rotation or twisting deformation, causing premature failure before its theoretical bending capacity, as determined using the undistorted shape, is reached. Further, while measures such as adding material of higher yield strength may increase theoretical bending support, they represent significant added costs, in many cases without yielding proportionate benefits or expected results.
In an effort to address the problems mentioned above, several improved truss embodiments are described in U.S. Pat. No. 6,079,165 sharing common inventors herewith. The embodiments involve various cross-sectional configurations intended to bring the elastic axis and shear center of the open truss section closer to the pole and to the point where load is transferred from the pole to the truss, thereby reducing torsional loading on the truss.
While the truss configurations described in U.S. Pat. No. 6,079,165 offer improved performance relative to prior trusses, there is still a tendency for all prior art trusses to rotate about the pole to a position where the load is no longer acting along an intended direction relative to the truss section, and is instead acting along a weak axis of the truss section. It has been observed that this problem actually gets worse as higher yield strength steel is used, thereby defeating the purpose of using higher yield steel. At the onset of yielding, there is a tendency for buckling to occur in pole-engaging side flanges of prior art trusses. Consequently, the geometry of the truss cross-section changes, thereby decreasing the effectiveness of the truss and leading to ultimate failure rather rapidly after the onset of first yielding. Generally speaking, prior art trusses have been designed for elastic capacity, and have not been designed to resist buckling.
Accordingly, there is a need for a pole reinforcement truss that better maintains its cross-sectional geometry after the onset of yielding.